Perhaps it might be more interesting to design a bitcoin checkbook, seeing as banknotes are designed to be reusable by many people and a BTC note with a private key more or less makes it anything but a banknote.

So why not instead a checkbook? Something where each check has the bitcoin private key obscured by some scratch off material like instant win lottery cards, and when you write this check, it tears out of the checkbook leaving a stub where you can keep track of transactions.

The back of the checkbook could contain a list of all the account numbers so you could easily check the balances. Or perhaps the producers of the check book could offer a web app where you can log in to your 'virtual checkbook' to keep track of what balances have been 'cashed'.

Something where each check has the bitcoin private key obscured by some scratch off material like instant win lottery cards,

This project, if I understand it correctly, is a "print-your-own Bitcoin banknote". I don't have a method of applying scratch-off material, and even if I did I would not want someone else figuring out how to defeat this defense without it being obvious my scratch-off layer had been tampered with, and then the notes continue to circulate even though a would-be criminal has begun accumulating the private keys. I couldn't prove that it wasn't me who was the first to redeem the code and transfer the funds back to myself.

There is one use case, however, that suggests a change that might be useful with these banknotes:

Space for a custom message. Like a memo field on a check.

If I am giving these out as a gift, printed on the back of a customer invoice, for instance, I know there will be many of these that will never be redeemed. Since I can keep a copy of the keys, I could reclaim any funds that were not redeemed after a certain expiration date. A memo field would allow me to print a "Redeem by: " date or other way to explain expiry.

I haven't read the whole thread, so maybe someone else already suggested this: don't be limited to paper only in the designs. Banknotes could also be printed on transparency film.

That way, QR-codes and privkeys can be broken up into grids of small squares and printed twice onto the banknote. One field contains half the squares, the second field the rest. You then need to fold the node so that the two layers create the complete code again. That way, a quick glance at the QR-code or the privkey isn't useful anymore.

Or print on two completely separate sheets. Store one in your safe, the other one somewhere else. You need both of them to redeem the coins then. Or it can be used as some sort of escrow: hand over sheet 1 and after the deal is done sheet 2 too. When the public code is on both, anybody can change the balance, but only when you have both sheets you can redeem it.

Using such plastic would also allow the user to cover the important codes with a paint that is soluble in water. Black paint covering black toner. You have to wipe it off to scan the redeem code for example.

I haven't read the whole thread, so maybe someone else already suggested this: don't be limited to paper only in the designs. Banknotes could also be printed on transparency film.

That way, QR-codes and privkeys can be broken up into grids of small squares and printed twice onto the banknote. One field contains half the squares, the second field the rest. You then need to fold the node so that the two layers create the complete code again. That way, a quick glance at the QR-code or the privkey isn't useful anymore.

Or print on two completely separate sheets. Store one in your safe, the other one somewhere else. You need both of them to redeem the coins then. Or it can be used as some sort of escrow: hand over sheet 1 and after the deal is done sheet 2 too. When the public code is on both, anybody can change the balance, but only when you have both sheets you can redeem it.

Using such plastic would also allow the user to cover the important codes with a paint that is soluble in water. Black paint covering black toner. You have to wipe it off to scan the redeem code for example.

Mad Magazine has those fold pictures. You could do that with a paper wallet. If you want the code to not be easily seen, you could print with invisible, UV, or infrared ink.

Any significantly advanced cryptocurrency is indistinguishable from Ponzi Tulips.

I haven't read the whole thread, so maybe someone else already suggested this: don't be limited to paper only in the designs. Banknotes could also be printed on transparency film.

That way, QR-codes and privkeys can be broken up into grids of small squares and printed twice onto the banknote. One field contains half the squares, the second field the rest. You then need to fold the node so that the two layers create the complete code again. That way, a quick glance at the QR-code or the privkey isn't useful anymore.

Or print on two completely separate sheets. Store one in your safe, the other one somewhere else. You need both of them to redeem the coins then. Or it can be used as some sort of escrow: hand over sheet 1 and after the deal is done sheet 2 too. When the public code is on both, anybody can change the balance, but only when you have both sheets you can redeem it.

Using such plastic would also allow the user to cover the important codes with a paint that is soluble in water. Black paint covering black toner. You have to wipe it off to scan the redeem code for example.

Mad Magazine has those fold pictures. You could do that with a paper wallet. If you want the code to not be easily seen, you could print with invisible, UV, or infrared ink.

Splitting the private key to front and back would increase security but would increase the rate of printing errors (imagine a single sided printer and multiple bills per page...multiple pages in a print job...you're just begging for mistake).

I haven't read the whole thread, so maybe someone else already suggested this: don't be limited to paper only in the designs. Banknotes could also be printed on transparency film.

That way, QR-codes and privkeys can be broken up into grids of small squares and printed twice onto the banknote. One field contains half the squares, the second field the rest. You then need to fold the node so that the two layers create the complete code again. That way, a quick glance at the QR-code or the privkey isn't useful anymore.

Or print on two completely separate sheets. Store one in your safe, the other one somewhere else. You need both of them to redeem the coins then. Or it can be used as some sort of escrow: hand over sheet 1 and after the deal is done sheet 2 too. When the public code is on both, anybody can change the balance, but only when you have both sheets you can redeem it.

Using such plastic would also allow the user to cover the important codes with a paint that is soluble in water. Black paint covering black toner. You have to wipe it off to scan the redeem code for example.

Mad Magazine has those fold pictures. You could do that with a paper wallet. If you want the code to not be easily seen, you could print with invisible, UV, or infrared ink.

I haven't read the whole thread, so maybe someone else already suggested this: don't be limited to paper only in the designs. Banknotes could also be printed on transparency film.

That way, QR-codes and privkeys can be broken up into grids of small squares and printed twice onto the banknote. One field contains half the squares, the second field the rest. You then need to fold the node so that the two layers create the complete code again. That way, a quick glance at the QR-code or the privkey isn't useful anymore.

Or print on two completely separate sheets. Store one in your safe, the other one somewhere else. You need both of them to redeem the coins then. Or it can be used as some sort of escrow: hand over sheet 1 and after the deal is done sheet 2 too. When the public code is on both, anybody can change the balance, but only when you have both sheets you can redeem it.

Using such plastic would also allow the user to cover the important codes with a paint that is soluble in water. Black paint covering black toner. You have to wipe it off to scan the redeem code for example.

Mad Magazine has those fold pictures. You could do that with a paper wallet. If you want the code to not be easily seen, you could print with invisible, UV, or infrared ink.

Splitting the private key to front and back would increase security but would increase the rate of printing errors (imagine a single sided printer and multiple bills per page...multiple pages in a print job...you're just begging for mistake).

This statement is false by definition: The definition of what is a concern for the purpose of my reward/bounty/contest is purely my choice, which may or may not be arbitrary. I am the judge, jury, and executioner in that respect.

But just to be clear, my concern was less "this uses too much ink", but rather that the ink cost doesn't add an offsetting artistic value. The choice that uses the most ink happens to spend that ink on a fractal-based image, which has a high level of visual appeal, invokes the semblance of visual features present in cash, and has good relevance to the discipline of mathematics being referenced here. I am not asking how many pixels are white, I am asking if the person who prints the note is going to get a good return on his ink investment, as measured in visual appeal.

That said, if you, for example, altered the curves behind Alan Turing's head and changed them to something that uses the ECDSA curve as a theme (discussed and illustrated earlier in the thread - it is a curve with points labeled P and a Q) the appeal will jump up quite a bit. And there IS a very real demand for an artistic banknote that uses the least possible ink - and you're doing fairly well in that respect with your most recent submission. If it were feasible, I'd much like a portrait of Turing made out of line art (sort of what's on submission #1), rather than a greyscale photograph.

If you're going to use Alan Turing, I would also add "(1912-1954)" to make clear we are honoring a dead pioneer of some sort, rather than what some might think is the "president" of the "Bitcoin corporation".

I am not quite sure which directions the notes are going. I could think of it in two ways:

1) notes that you produce yourself for purposes of storage.2) notes that you produce yourself but intend to give away as payment.

The general problem to me, it seems, is the following: A potential receiver of the note could probably look up the amount of BTC related to the pubkey in the blockchain, but this would urge him or her to go "digital" again. The second problem is that when you are producing notes to give away, how are you going to ensure the private key on the note actually belongs to the public key printed on the note? Anyone accepting BTC in paper would have to trust you, or, in case of the website mentioned in this thread, offering paper bills with sealed private keys, the website producing them.

IDEA for 1) If we think of paper bills as means of storage, designers may want to implement steganography in order to protect the private key. I am not sure if there is a meaningful way to hide a private key (or merely an AES-encryption key to the private key) in the bill. Furthermore (I don't know if it is feasible) you could think of some type of steganographic algorithm that can be made public whilst leaving it to the owner of the bill to think of a distinctive pattern to remember and thus being sure, that he is the only one in charge of the private key. (In my head there is a picture of me in the future finding a bunch of paper notes that I created in the past and wondering if the amount printed on them is still available - if the private key is on them in clear text, you can never be totally sure.)

IDEA for 2) There are two things we are aware of. One is: There is no central authority in bitcoin that we can (or would even want to) put trust in. There has to be some degree of trust between business partners. The second is: Paper bills should be a kind of debt, but in practice they are not. Each $50 note should be a debt that someone has to pay. And WHEN it has been payed, what would consequently have to follow, is the extinction of the respective note. Let me clarify this for those who don't see my point right away: I work for someone for two hours and he offers me to help me for 2 hours when I need help. In the meantime we set up a tiny contract (a bank note) saying that he owes me a 2 hours' work. If I find myself in a situation where I receive help from a third person I might consider just "forwarding" my entitlement to two hours of work to this very person. With my tiny contract, I just created money - as long as people trust in my original "employee" to pay his debt and work the two hours for a random person. When this has been done, the contract is fulfilled and needs to be extinct. It does not happen to bank notes though, but we could make it happen to bitcoin notes.

Think of this: When you do online transactions using bitcoin on bitcoin-otc you have to build up trust. You can either do this by providing your positive transaction record (and primarily you'd go this way), but secondly you could just reveal your true identity to the person you are dealing with. This is pretty hard on the internet, because even a GPG-key that is well linked in the web of trust may be fake or might not be accepted in court, or you may provide your business partner with a fake address...In the drafts for bank notes that we saw so far there were some having honorable persons on it. What Washington, Lincoln or whoever looks at us from our dollar notes wants to evoke is trust in the bill. Why don't we consider replacing their faces with ours? Go print a note with a public and private key plus your face on it and thus assuring the recipient that you vouch for the amount printed on it? You would not even have to go this far. Take one of the excellent drafts in this thread and add a line for MY NAME: ____ and SIGNATURE: ____. I suppose a lot of you won't like this, because you'd have to reveal your identity. But: You are only revealing it to the person your are paying. And once it has been payed, the buyer will act reasonably if he transfers the BTC on it to his own wallet because he cannot assume that he can just forward the note to the next recipient, because this very recipient will certainly (want) to put more trust in the person that is handing over the note than in the person that once created it. Therefore, the note becomes useless and can be destroyed.

Of course you could think of several attacks to this idea. One is that you could forward a personalized bill claiming to be the creator when actually you are not. This would call for id checking which is not the way to go (you might want to reconsider putting your face on the note :-) Another one is for the malicious creator of a note to spend the coins right before you hand it over to your business partner and then urge him to instantly look up the amount an the blockchain. As it will not have been processed after very few minutes, the recipient cannot be sure. I guess, though, that there are solutions to these problems that we could elaborate on. This does not need to happen here. At this place I just want to put in front the idea of personalized paper bills. It is basically just like GPG-signing private/public-keys on the internet to reveal your id to a single business partner.

Splitting the private key to front and back would increase security but would increase the rate of printing errors (imagine a single sided printer and multiple bills per page...multiple pages in a print job...you're just begging for mistake).

No. You are splitting the code, but you print both parts at the same time, onto the same film. Or in case of "split" notes, on two different films. You do not print onto the backside. That's why the medium is transparent, so there is no need to do anything on the backside. You are still thinking of paper.That said, in case of the private key in text form, you can use a monospaced font and print every second letter in area 2 (or film 2). So even if you somehow mess up the OR code, you still can type in the privkey.

What are folds going to protect against? Someone snapping a picture of the bill and redeeming it? Why, assuming this were to happen, they could easily splice it back together with Photoshop. It's not as though the information isn't there for an attacker to redeem the bill, he just has to work a little bit to piece it together.

Companies claiming they got hacked and lost your coins sounds like fraud so perfect it could be called fashionable. I never believe them. If I ever experience the misfortune of a real intrusion, I declare I have been honest about the way I have managed the keys in Casascius Coins. I maintain no ability to recover or reproduce the keys, not even under limitless duress or total intrusion. Remember that trusting strangers with your coins without any recourse is, as a matter of principle, not a best practice. Don't keep coins online. Use paper or hardware wallets instead.

What are folds going to protect against? Someone snapping a picture of the bill and redeeming it? Why, assuming this were to happen, they could easily splice it back together with Photoshop. It's not as though the information isn't there for an attacker to redeem the bill, he just has to work a little bit to piece it together.

The stitching would require additional work and could not be AUTO DECRYPTED using built in QR software. So accidental spends would be somewhat avoided.

Think of it as a PIN to a debit card. Requires a little WORK to spend the money.